This invention relates to a method and apparatus for measuring the frequency of a series of pulses. There are a number of situations in which it is desirable to measure the cadence or rate of occurrence, i.e. the frequency, of a train of pulses having a usually regular spacing therebetween. Examples of such pulses are the human heartbeat, engine sounds, or in the successive impacts of one machinery part upon another.
Measurement of the frequency of the human heartbeat is routinely undertaken during the course of physical examinations. During such examinations the physician uses a stethoscope to listen to the sounds of the heart and other areas of the patient's body, and while listening to the heart, the physician commonly measures the pulse rate by counting the number of pulses heard within a timed interval during which his attention is focused on his watch or other time measuring means.
A variety of instruments have been developed in the prior art for the measurement of the pulse frequency of humans, ranging from mechanical devices, through electronic listening devices, to the present day sophisticated EKG instrumentation which provides continuous digital readout of the pulse frequency while recording an EKG tracing.
U.S. Pat. No. 1,675,799 discloses a mechanism intended to indicate the pulse frequency of a human being by the use of oscillating bodies consisting of timed frequency tongues, flywheels or pendulums, U.S. Pat. No. 2,831,479 discloses an instrument which utilizes a microphone adapted to be placed on the chest of a patient, a timer presettable to measure a predetermined time interval, and an electromechanical pulse counter operable to indicate the number of pulses detected through the microphone during the interval for which the timer is set.
Other listening type devices of the prior art include the heartbeat frequency analyzer disclosed in U.S. Pat. No. 3,171,406 which electronically indicates the frequency components of a heartbeat to permit early detection of a heart malfunction; the acoustic and electronic stethoscope disclosed in U.S. Pat. No. 3,247,324 which provides amplification of a selected range of frequencies; and the stethoscope with display disclosed in U.S. Pat. No. 3,858,005 wherein the heart or other body sounds are converted to electrical signals which are amplified and fed to a cathode ray tube for visual display of such sounds.
EKG type instrumentation has also been the subject of developments concerned with human heartbeat rate monitoring. U.S. Pat. Nos. 3,595,219 and 3,613,670 disclose EKG tupe instruments which provide audible or visual signals when the heartbeat rate of the patient is below or above a predetermined rate or rate range.
U.S. Pat. No. 3,830,227 discloses a portable electrocardiophone which produces an audible tone pattern which can be audibly analyzed to identify and diagnose various known types of heart arrhythmias. This patent also makes reference to previously developed portable devices which produce a flashing light or audible beep to represent the heartbeat rate. U.S. Pat. No. 3,455,293 discloses a stethoscope providing for electrical connection between the pickup head thereof and an associated EKG machine, which connection permits the physician to listen to a patient's heart at the same time an EKG tracing of the heart action is made.
In spite of all of the developments in stethoscopes and heart rate monitoring instrumentation which have taken place, no apparatus has been developed which provides the functions and portability of a stethoscope and by which pulse frequency can be measured without requiring the physician to count pulses or measure the time.
It is therefore a general object of the present invention to provide an improved stethoscope by the use of which a physician can measure pulse frequency without having to count pulses or measure the time.
Another object of the invention is to provide an improved stethoscope which is simple to use and, in fact, can be used by medically untrained personnel, including the patient himself.
A further object of the invention is to provide an improved stethoscope which in one form thereof permits measurement of pulse frequency by listening to audible signals and reference impulses, and in another form thereof permits such measurement by observation of visual signals and reference impulses, the features of such stethoscopes which permit such measurement, in neither form, interfering with the normal use of such stethoscope.